A nanofiber is an ultra-fine fiber having a diameter of 1-800 nm, and has various physical properties that cannot be gained from a conventional fiber. Accordingly, a web, composed of the nanofiber, as a membrane type porous material may be usefully applied to various fields, such as filters, wound dressings, artificial supporters, defensive clothes against biochemical weapons, separation membranes for secondary batteries, and nanocomposites.
A representative example of a conventional process of producing the nanofiber includes an electrospinning process where a raw material solution of a fiber is spun while being charged to produce a fiber having a very small diameter. Examples of the production of the nanofiber using the electrospinning process are disclosed in Korean Pat. Laid-Open Publication Nos. 2000-11018, 2003-3925, and 2003-77384, and U.S. Pat. No. 6,183,670. However, the nanofiber produced according to the conventional electrospinning process is limited to a non-woven fabric type. With respect to this, Doshi et al. assert that nanofibers are produced in a form of nanoweb, that is, non-woven fabric, because, when drops, which consist of a polymer solution and are formed at a tip of a spinneret, burst by an applied high voltage and are then collected on a collector to produce the nanofibers through the conventional electrospinning process, the nanofibers are anisotropically oriented in the collector [Doshi and Reneker, “Electrospinning Process and Application”, Journal of Electrostatics, 1995, 35, 151-160].
Furthermore, they point out a problem that since the non-woven fabric type nanofiber consists of a single fiber, while the drops, which are formed at the tip of the spinneret during the electrospinning process, are spun toward the collector at a critical voltage (Vc), the single fibers collide with each other before the drops reach the collector. Consequently, the single fibers are interfered or combined with each other, causing conglomeration. With respect to this, Korean Pat. Laid-Open Publication No. 2002-50381 discloses the production of a nanofiber employing a copolymer of polyethylene terephthalate and polyester, instead of a single component, as a spinning solution through a conventional electrospinning process. However, that nanofiber does not break from a non-woven fabric type, either. The non-woven fabric type nanofiber has very poor mechanical strength. Particularly, if the nanofibers are intertwined into a thread, undesirably, an additional connection fiber is required to connect the single fibers to each other, and the final thread is readily broken. Accordingly, there remains a need to improve the non-woven fabric type nanofiber so as to be applied to various fields.
Therefore, the present inventors have conducted studies into the production of a nanofiber capable of being applied to various fields, resulting in the finding of the following fact. In a procedure of producing the nanofiber where drops, which consist of a polymer solution and are formed at a tip of a spinneret, burst by an applied high voltage to be collected on a collector according to a conventional electrospinning process, after the polymer solution is spun to one or more first collectors to produce the nanofiber, the nanofiber, which is collected on the first collector, is recollected onto a second collector to be continuously collected thereon, thereby continuously producing a filament type nanofiber having mechanical properties that are better than a conventional nanofiber. Based on the above finding, the present inventors accomplished the present invention.